Orthogonal frequency division multiplexing (OFDM) is used in wireless communication systems, such as IEEE 802.11 (WiFi) systems, IEEE 802.16 (WiMAX) systems, and systems that use the Long Term Evolution (LTE) specifications of the Universal Mobile Telecommunications System (UMTS). In the OFDM approach, data is organized into symbols that are used to modulate a plurality of sub-carriers. The sub-carriers are spaced apart in frequency by a subcarrier spacing, Δf.
In order to reduce or eliminate inter-symbol interference (ISI) that may be caused by multipath propagation, a portion of each symbol (typically the final portion) is duplicated and appended to the beginning of the symbol as a cyclic prefix. This approach can be effective, provided that the length of the cyclic prefix, TCP, is greater than the delay spread, TD, that is caused by multipath propagation. Thus, a longer cyclic prefix can beneficially make an OFDM signal more robust against multipath propagation. However, this benefit comes at a cost. Because the cyclic prefix is redundant information, a longer cyclic prefix for a given OFDM symbol duration results in more overhead and less spectral efficiency. Although it is possible to mitigate the inefficiency caused by a longer cyclic prefix by increasing the OFDM symbol duration, TU, this requires a reduction in the subcarrier spacing by the relationship Δf=1/TU. A smaller subcarrier spacing, however, makes the OFDM signals more sensitive to frequency offsets, such as Doppler shifts, which can cause inter-carrier interference (ICI).
To balance these competing considerations, LTE specifications define three different parameterizations with three different cyclic prefix lengths for downlink communications. The basic parameterization uses a cyclic prefix length of approximately 5 microseconds. In addition, LTE specifications define a parameterization with an extended cyclic prefix length of approximately 17 microseconds. This extended cyclic prefix length is intended for larger cells (such as cells used in rural areas), which may be expected to have a larger delay spread. LTE also supports a multi-cell broadcast transmission mode with a cyclic prefix length of approximately 33 microseconds.